Late summer/early fall grape disease control; 2016

By: Bryan Hed

We’re in the final leg of the season and it’s time to size up our remaining challenges through the ripening period. Fruit are no longer susceptible to many of the major diseases like powdery and downy mildew and black rot that can cause crop loss during earlier stages of berry development. But for some grape varieties, particularly wine grapes that produce compact clusters, there is another major hurdle to work through to harvest; late season bunch/sour rot. I am referring to the rotting of fruit in clusters that occurs during the later stages of the ripening period, just a few heartbreaking days or weeks before harvest. Bunch rot can involve the colonization of fruit by a number of different microorganisms, both fungi and bacteria. But the main culprit in most regions of the Northeastern U.S. is the fungus, Botrytis cinerea (Figure 1). Fortunately, we have a number of chemical control options that are quite effective against this fungus that I have listed below. I have organized them according to the FRAC (Fungicide Resistance Action Committee) group that each product belongs to. Basically FRAC groups are fungicide chemistries with the same or similar mode of action, so that pathogen resistance to one fungicide is going to confer cross resistance to another, within that same FRAC group. For example, notice that Vangard and Scala are in the same FRAC group; 9. This means that if a population of Botrytis in a vineyard has developed resistance to the active ingredient in Vangard, then it will also be resistant to the active ingredient in Scala, even though the active ingredients may be different (cyprodinil in Vangard and pyrimethanil in Scala). The mode of action (the way in which the fungicide disrupts a specific metabolic pathway in the fungus, killing it) of these two chemistries is the same, or similar enough that pathogen resistance to one chemistry will confer resistance to the other.

FRAC group 2: Rovral, 7 day pre-harvest interval

FRAC group 7: Endura, 14 day pre-harvest interval

FRAC group 7 (and 3, which is not for Botrytis): Luna Experience, 14 day pre-harvest interval

FRAC group 7 and 11: Pristine, 14 day pre-harvest interval

FRAC group 9: Vangard, Scala, 7 day pre-harvest interval

FRAC group 9 (and 3, which is not for Botrytis): Inspire Super, 14 day pre-harvest interval

FRAC group 9 and 12: Switch, 7 day pre-harvest interval

FRAC group 11: Flint, 14 day pre-harvest interval

FRAC group 17: Elevate, 0 day pre-harvest interval

No doubt many wine grape growers have already applied a bloom, pre-bunch closure, and veraison spray to bunch rot susceptible varieties. However, one or more applications may be necessary in some vineyards. Populations of the Botrytis fungus are quite adept at developing resistance to these fungicides; be mindful to rotate FRAC groups and limit the application of any one FRAC group to one or two per season to delay the development of that resistance. If you have to use a FRAC group more than once per season, it would be better to compose one of those two applications with a material that contains a second FRAC group for Botrytis. For example, if you already used Scala, it would probably be better to apply Switch (after you’ve already rotated to FRAC group 2, 7, 11, or 17) than to apply Vangard or another Scala spray. Most of these materials are considered ‘high risk’ for resistance, so rotation is extremely important to maintaining the effectiveness of these products. Also, pay attention to pre-harvest intervals which range from 0 to 14 days. That said, you can’t spray your way completely out of the damage that Botrytis and other microorganisms can cause; consistently effective bunch rot control programs must be integrated with a generous dose of cultural practices like fruit zone leaf removal, sanitation, canopy management, and vine balance. And, unfortunately, these chemistries listed above are specific for Botrytis and will not control many of the other microorganisms that may make up the bunch rot complex or that lead to the dreaded sour rot complex.

Figure 1. Botrytis cinerea sporulating on damaged grapes of Vitis interspecific hybrid ‘Vignoles’. Such damage often occurs as a result of berry overcrowding in overly compact clusters. The damage leaves fruit open to colonization by the ever present Botrytis fungus and by many other fruit rot organisms.

I’ve already alluded to one of the major predisposing factors for bunch rot (including sour rot) in grape clusters, and that is cluster compactness. The compactness of clusters is responsible not only for initiating much of the fruit rot that occurs in clusters, but perhaps more importantly, for the rapid spread of rots throughout the cluster (Figure 2). Rots can be initiated in loose grape clusters as well (by bird or insect damage, for example), but generally do not spread beyond the damaged berry or berries. However, in compact clusters, a single damaged berry can spread rot to large sections of the cluster by virtue of the close contact between those berries. Contact between berries in compact clusters also reduces cuticle thickness, an important barrier to rot pathogens, and reduces pesticide penetration into clusters for protection of berry surfaces against Botrytis and damage by insects. Cluster compactness also increases the effects of retained bloom trash (dead flower parts) inside clusters that can provide a substrate for Botrytis, increasing fruit rot by harvest. Taken together, this generally makes berries in compact clusters much more susceptible to invasion by fruit rot pathogens than berries in loose clusters.

A series of greenhouse experiments we conducted years ago also suggested that latent (dormant) infections of Botrytis can be activated by the kind of berry injury that occurs in compact clusters. Latent Botrytis infections are infections that occur during bloom and the early fruit development period for which you apply that bloom and pre-closure spray. Years ago, we monitored the incidence of latent infections in our block of Vignoles and found that even though the incidence appeared to increase throughout the berry development period, most of these infections did not lead to fruit rot by harvest. In fact, when we inoculated clusters of potted, greenhouse grown Chardonnay vines with Botrytis shortly after bloom, generating high levels of latent infection in berries, the berries did not rot during ripening if they remained intact in the greenhouse, unexposed to weather, birds, insects, or compactness (the clusters were thinned after inoculation and thinned berries were used to determine latent infection levels). However, when we surface sterilized the berries (to eliminate any Botrytis on the outside of berries) and created small injuries at the berry/pedicel interface of ripe berries (the kind of injury that commonly occurs in overcrowded clusters) the vast majority of the inoculated berries quickly rotted compared to berries that were not inoculated with Botrytis (checks).

By loosening clusters, damage from berry overcrowding can be minimized and bunch/sour rot development can be greatly alleviated. Unfortunately, loosening clusters in a consistently effective AND cost effective way is not always an easy thing to accomplish. Over the years we have examined a number of potential methods for cluster loosening with varying levels of success. Treatments such as pre-bloom fruit zone leaf removal have provided the most consistently significant reductions in cluster compactness and fruit rots in most years. The pre-bloom timing of fruit zone leaf removal simply combines the benefits of an open, sun lit fruit zone (which has been well documented by many investigators over the past several decades) with a reduction in cluster compactness and rot susceptibility. In our experiments, this treatment has typically been applied by hand, but the technology exists to mechanically remove leaf tissue around inflorescences (pre-bloom) without serious damage to them, and trials are being conducted to evaluate the mechanization of the pre-bloom leaf removal on a number of grape varieties. So far, results have been mixed depending on variety and trellis training system. In vineyards where we were able to compare pre-bloom mechanized leaf removal with pre-bloom leaf removal by hand and post-bloom mechanized leaf removal, the effects of pre-bloom mechanized leaf removal (increased light exposure of clusters, looser clusters, less rot, yield reduction) generally fell somewhere between the two latter treatments. The hope of this research is to expose growers to some new possibilities for fruit rot control and increase the potential for its adaptation to commercial vineyards and adoption by growers. We’ve examined other technologies with potential for cluster loosening and improved fruit rot control, but unfortunately their adoption is more problematic. For example, we have found that inexpensive gibberellin sprays around bloom have also been effective at loosening clusters and enhancing rot control on Vignoles and Chardonnay with little or no serious negative side effects. But they are currently ‘off label’ and are very unlikely to ever become legal applications in the United States. Also, the effects of gibberellin sprays are variety specific and therefore must be examined and defined for each variety: in our experience, low rates (5-20 ppm) can have serious negative side effects on Vitis vinifera Riesling, whereas rates as high as 100 ppm have had little or no effect on Vitis interspecific hybrid ‘Chancellor’.

Figure 2. Botrytis bunch rot. The compactness of these bunches has contributed to rapid and severe rotting of large portions of these clusters (left). Loose clusters of the same variety are far less affected by the spread of rot within the bunch (right).

More recently, work conducted by Megan Hall, a grad student of Wayne Wilcox at Cornell University, has shown that additional pesticide applications during the latter stages of ripening can significantly reduce the development of sour rot. Her work has shown a close connection between fruit flies and sour rot development; the presence of the flies is important to the accumulation/generation of acetic acid in rotting fruit. Treatments composed of weekly, tank mix applications of an insecticide (to control the flies) and an antimicrobial (to kill bacteria) have been found to reduce sour rots by 50-80% over unsprayed vines. So far, the best results appear to occur when weekly sprays are initiated before sour rot symptoms are observed (preventive sprays before about 15 brix). This exciting work should provide yet another effective option for sour rot control in the wet, humid parts of the eastern U.S. and we are looking forward to hearing more about this rot control option in the near future.

LATE SEASON LEAF DISEASE CONTROL

Beyond the management of bunch rot on susceptible wine varieties, there is also the matter of keeping canopies (leaves) as clean and functional as possible, for as long as possible. Diseases like powdery and downy mildew can continue to be of concern into late summer and early fall, especially for growers of Vitis vinifera. The mildews can greatly reduce leaf function if allowed to spiral out of control. The ability of the canopy to continue to photosynthesize is crucial to the ripening of the crop and canes and the storage of sugars (starch) in trunks, arms, and roots, which relates to winter hardiness. The winters of 2014 and 2015 are harsh reminders of just how important this can be. Allowing grapevines to go into winter dormancy with less than optimal preparation can leave them more susceptible to damage by severe cold and another plague of crown gall to have to deal with for years to come.

Good control of powdery mildew up to about Labor Day can also go a long way to reducing overwintering inoculum and disease pressure the following spring. This finding was the result of some excellent research conducted by Wayne Wilcox, Dave Gadoury and graduate students at Cornell University. When powdery mildew infected leaves die by that first hard frost in fall, the mildew on those leaves stops developing and also dies…unless it has had time to form fully mature, winter resistant resting structures called chasmothecia. If the chasmothecia in a powdery mildew colony do not have time to fully mature before the grape tissue dies (as from infections that were roughly initiated after early September), they will not survive the dormant period (winter) and will not contribute to the bank of primary inoculum that infection periods draw upon the following spring. Knowing this, a grower can get a better handle on the ‘size’ of the powdery mildew problems he/she will potentially face next spring. If, for example, you had heavy mildew development earlier in this season (on clusters and/or leaves), expect to have to deal with powdery mildew early next season and take appropriate action during early shoot growth stages with preventive fungicide sprays. This is particularly important if you are growing Vitis vinifera and much less important for growers of native varieties like Concord and Niagara.

Downy mildew appears to be much less a widespread problem this year. In fact, in our ‘neck of the woods’ along the southern shore of Lake Erie, droughty conditions have prevailed throughout most of the season, and only now are we even beginning to see a few downy mildew infections on leaves close to the ground. At this point in the season regular scouting for this disease is the first line of defense, and in areas that remain relatively dry, perhaps the only control measure needed (?). However, in areas where the disease has remained active throughout the season, be vigilant about keeping it under tight control. Late season epidemics of this disease can quickly strip susceptible wine varieties of their leaves, effectively bringing an early halt to ripening.